Referring to FIG. 7, a torque converter generally comprises a bowl-like input housing 1 which is connected to a flywheel (not shown) of an engine. An outer peripheral end portion 2 of the housing 1 is welded to an outer peripheral end portion 5 of a shell 4 of a pump impeller 3. The input housing 1 and the shell 4 form an outer wall of the torque converter, in which a turbine runner 6, a stator 7 and a lock-up clutch 8 are disposed.
In the known structure, the shell 4 is provided at the end portion 5 with an outer peripheral surface 10 having a small diameter and an axial length L. This outer peripheral surface 10 is fitted into an inner peripheral surface 11 formed at the end portion 2 of the housing 1, and the free end of the end portion 2 and the base end of the surface 11 is welded together, as indicated at 12.
However, in the structure in FIG. 7, the outer peripheral surface 10 is of merely cylindrical shape, and a space 13 extending in the axial direction (right to left in FIG. 7) is formed between the outer surface 10 and the inner surface 11. In the welding process, spatter generated at the portion 12 may pass through the space 13 into the internal space in the torque converter. The spatter in the internal space causes wear or abrasion of a friction facing of the lock-up clutch 8, resulting in short durability thereof. Further, wear powder generated from the facing may enter into an automatic transmission disposed subsequent to the torque converter, which causes choke or clog of a filter in a hydraulic line of a hydraulic pump, which is a control power source of the torque converter.
Of course, the entry of the spatter can be prevented by eliminating the space between the outer and inner surfaces 10 and 11. However, in such a structure, thermal strain resulting from the welding causes deviation or error in the axial positions of internal parts and members. Therefore, it is essential to form the space 13 having an appropriate radial length, e.g., of 0.4 mm-0.5 mm, between the surface portions 10 and 11.